produced by Cooling Gases to Low temperatures. 537 



or less the same amount of moisture (if any), and yet, as has 

 been shown, the " critical temperatures" range from -—70° C. 

 to below -175°C. 



The effects cannot therefore be due primarily to the gases 

 experimented upon not being perfectly dry. Nor can the 

 effects obtained in air and oxygen be due to traces of C0 2 , 

 for we may again reasonably suppose the gases derived from 

 boiling liquid air and liquid oxygen to be as free from C0 2 as 

 hydrogen prepared in the way described, yet the "critical 

 temperature " for hydrogen is much lower than for air and 

 oxygen. This view is strengthened by the fact that the 

 " critical temperature " for air mixed with nearly 4 per cent, 

 of C0 2 was raised only some 20 degrees. 



(3) It might also be questioned if the gas had thoroughly 

 warmed up before admission into the cloud chamber, for the 

 entrance of cold gas into the moist gas in the latter might 

 conceivably of itself produce nuclei. To invalidate this con- 

 tention, it is only necessary to point out that hydrogen cooled 

 to — 1 50° C. say, would not have had as good a chance to regain 

 atmospheric temperature in the 90 seconds interval allowed 

 as C0 2 cooled to —80° C. ; yet no effect is produced in the 

 former, and dense clouds in the latter case. The fact that 

 showers can be obtained on admitting the gas into the cloud 

 chamber ten minutes after the cooling agent is removed still 

 further disarms this criticism. 



Discussion of the Results. 



We are by no means sure as to the correct explanation of 

 the origin of the nuclei produced in a gas, as we have 

 described, by severe cooling. As a possible explanation of 

 the effects obtained in hydrogen, air, oxygen, and nitrogen, we 

 tentatively propose the view that when gases are cooled to a 

 sufficiently low temperature, yet considerably higher than 

 their real liquefying points, the molecules of the gas come 

 together and form aggregations of a considerable size which 

 in some way or other (at present very difficult to under- 

 stand) are able to persist for a long time after the gas has 

 regained its normal temperature. In other words, the effects 

 suggest the interesting fact that what we might call incipient 

 liquefaction occurs in these gases at temperatures well removed 

 from their real liquefying points. It may also be pointed out 

 that, broadly speaking, the lower the real liquefying point of 

 a gas, the lower is its "critical temperature/'' and the greater 

 is the temperature-distance between the '■' critical temperature " 

 and the real liquefying point. The results for nitrogen at a 



